ShyTorque is right that the primary aim was to minimise the exposure time to intruders in the vicinity of the a/f. But the R&B has the additional peacetime benefit of being an easy way to slot a formation into the visual circuit.
On the point of being prohibited below 1000' at Manston, R&B heights have been going up steadily since the original purpose was forgotten or became unnecessary. When I did my acceptance check on my first Canberra Sqn, my boss (flying as my No2) bollocked me for being above 250' when I broke! changed days, eh?